The Faraday effect is a magneto-optical effect resulting from interaction between an electromagnetic wave and a material medium in the presence of a magnetic field.
When subjected to a magnetic field in the same direction as the electromagnetic wave, some material mediums present optical activity in the form of induced, non-reciprocal circular birefringence, i.e. by propagation speeds in the medium which are different for righthand and lefthand circular polarization.
Implementing the Faraday effect for measuring an electric current is done either by means of a linearly polarized light wave which may be considered as being the sum of two circularly polarized light waves, one having righthand polarization and the other lefthand polarization, with the phase shift in the direction of the polarization plane being measured by polarimetric or interferometric techniques, or else by means of two counter-propagating light waves which are both circularly polarized in the same lefthand or righthand direction and with the phase shift being measured by the so-called Sagnac interferometric technique.
In both cases, i.e. a single linearly polarized light wave or two counter-propagating circularly polarized light waves, the phase shift angle .delta..PHI. due to the Faraday effect is proportional to the instantaneous value I of the current to be measured, with a coefficient of proportionality k1 referred to herein as the Faraday effect efficiency factor: EQU .delta..PHI.=k1 I=a V N I
where a is equal to 1 for a linearly polarized wave and to 2 for counter-propagating circularly polarized waves, V being the Verdet constant, and N being the number of turns travelled by the light waves around the conductor carrying the current to be measured.
Interferometric techniques which make use of beats between two light waves give rise to a signal at the output from a photodetector which is a cosine function of the instantaneous current to be measured since it is proportional to the light power P which results from adding two light waves, as given by: EQU P=P1+P2+2.sqroot.P1.P2 cos .delta..PHI.
(where P1 and P2 are the respective optical powers to the two light waves), in other words: EQU P=P1+P2+2.sqroot.P1.P2 cos k1I
This output signal can be written in the general form: EQU ko+k2' cos k1I
where ko is a constant and k2' is a coefficient called the scale factor. This gives rise to a variation law having zero sensitivity in the small current range which, in practice, is the range where highest accuracy is sought. In order to avoid this drawback, attempts have been made to modify the interferometry conditions in order to obtain an output signal which obeys a sine law.
For further detail, reference may be made to French patent application No. FR-A-2 461 956 which describes measuring apparatus for measuring an electric current by the Faraday effect and implemented in a Sagnac interferometer having an output signal with a component whose amplitude is a sine function of the instantaneous value of the current to be measured. This property in the output signal is obtained by modulating the phase shift angle between the two counter-propagating light waves running round the interferometer loop, thereby giving rise to birefringence phenomena in the interferometer which are either non-reciprocal (by modulating a current flowing through an auxiliary winding) or else are reciprocal but pulsed at half the resonance frequency of the interferometer loop (referred to as the natural frequency of the interferometer), so as to give rise to instantaneous phase differences which are equal but opposite in sign on the two counter-propagating waves by means of the elasto-optical effect or of the elasto-acoustic effect.
The addition of these birefringence phenomena (whether they be reciprocal or not) has the effect of transforming the expression for the light power P given by the two light waves leaving the interferometer, as follows: EQU P=P1+P2+.sqroot.P1.P2 cos [.delta..PHI.+2.alpha. cos (2.pi.ft+.beta.)]
This light power P has a frequency spectrum which is rich in components, and in particular it includes:
a direct component;
a component at the modulation frequency f proportional to sin .delta..PHI. and having maximum variation in the small current range; and
a component at twice the modulation frequency f which is proportional to cos .delta..PHI. and which cancels with the current to be measured.
The component of the frequency f in the output signal from the interferometer is separated from the others and is synchronously demodulated in order to give a signal having the form: EQU k2 sin (k1 I)
where k2 is a new scale factor. This scale factor k2 suffers, as does the preceding scale factor k2', from the drawback of being sensitive to drift in the various components in the measurement chain, and in particular to variations in the light power emitted by the light source delivering the beam from which the two counter-propagating waves running through the interferometer loop are derived, thereby giving rise to considerable inaccuracy in the measurement.
In order to solve this difficulty, proposals are made in the above-mentioned French patent to use a zero method by subjecting the interferometer loop to a reference magnetic field generated by an auxiliary coil carrying a reference current which can be adjusted in such a manner as to compensate for the phase shift .delta..PHI. induced by the Faraday effect on the current to be measured.
This zero method suffers from the drawback of being very difficult to implement in current measuring step-down apparatus for use with high tension currents to be measured because of the difficulty in making the auxiliary coil. The product of the number of optical fiber turns multiplied by the number of electrical conductor turns in said auxiliary coil is proportional to the step-down ratio which must exist between the current to be measured and the reference current, which ratio is of the order of 100,000.
The object of the present invention is to remedy measurement inaccuracy due to drift in the scale factor without using a zero method, in order to make it possible to make step-down measurement apparatus for measuring high tension current at a price which is not prohibitive.